text: An important class of complexes that violate the 18e rule are the 16-electron complexes with metal d configurations. All high-spin d metal ions are octahedral (or tetrahedral), but the low-spin d metal ions are all square planar. Important examples of square-planar low-spin d metal Ions are Rh(I), Ir(I), Ni(II), Pd(II), and Pt(II). At picture below is shown the splitting of the d subshell in low-spin square-planar complexes. Examples are especially prevalent for derivatives of the cobalt and nickel triads.  Such compounds are typically  square-planar. The most famous example is Vaskas complex (IrCl(CO)(PPh)), [PtCl], and Zeises salt [PtCl(η-CH)].  In such complexes, the d</sub> orbital is doubly occupied and nonbonding.
  Many catalytic cycles operate via complexes that alternate between 18-electron and square-planar 16-electron configurations.  Examples include Monsanto acetic acid synthesis, hydrogenations, hydroformylations, olefin isomerizations, and some alkene polymerizations.
  Other violations can be classified according to the kinds of ligands on the metal center.